diff --git a/kitty/core_text.m b/kitty/core_text.m
index 12ea6f489..8990a4086 100644
--- a/kitty/core_text.m
+++ b/kitty/core_text.m
@@ -422,7 +422,7 @@ static struct { char *buf; size_t capacity; } ft_buffer;
 static CFStringRef
 lc_as_fallback(const ListOfChars *lc) {
     ensure_space_for((&ft_buffer), buf, ft_buffer.buf[0], lc->count * 4 + 128, capacity, 256, false);
-    cell_as_utf8_for_fallback(lc, ft_buffer.buf);
+    cell_as_utf8_for_fallback(lc, ft_buffer.buf, ft_buffer.capacity);
     return CFStringCreateWithCString(NULL, ft_buffer.buf, kCFStringEncodingUTF8);
 }
 
diff --git a/kitty/line.c b/kitty/line.c
index ec127bd43..4875005ef 100644
--- a/kitty/line.c
+++ b/kitty/line.c
@@ -414,13 +414,13 @@ cell_as_unicode_for_fallback(const ListOfChars *lc, Py_UCS4 *buf, size_t sz) {
 }
 
 size_t
-cell_as_utf8_for_fallback(const ListOfChars *lc, char *buf) {
+cell_as_utf8_for_fallback(const ListOfChars *lc, char *buf, size_t sz) {
     char_type ch = lc->chars[0] ? lc->chars[0] : ' ';
     bool include_cc = true;
     if (ch == '\t') { ch = ' '; include_cc = false; }
     size_t n = encode_utf8(ch, buf);
     if (include_cc) {
-        for (unsigned i = 1; i < lc->count; i++) {
+        for (unsigned i = 1; i < lc->count && sz > n + 4; i++) {
             char_type ch = lc->chars[i];
             if (ch != VS15 && ch != VS16) n += encode_utf8(ch, buf + n);
         }
diff --git a/kitty/lineops.h b/kitty/lineops.h
index 70b362e8e..f917fdb5b 100644
--- a/kitty/lineops.h
+++ b/kitty/lineops.h
@@ -79,7 +79,7 @@ index_type prev_char_pos(const Line *self, index_type x, index_type num);
 bool line_as_ansi(Line *self, ANSILineState *s, index_type start_at, index_type stop_before, char_type prefix_char, bool skip_multiline_non_zero_lines) __attribute__((nonnull));
 unsigned int line_length(Line *self);
 size_t cell_as_unicode_for_fallback(const ListOfChars *lc, Py_UCS4 *buf, size_t sz);
-size_t cell_as_utf8_for_fallback(const ListOfChars *lc, char *buf);
+size_t cell_as_utf8_for_fallback(const ListOfChars *lc, char *buf, size_t sz);
 bool unicode_in_range(const Line *self, const index_type start, const index_type limit, const bool include_cc, const bool add_trailing_newline, const bool skip_zero_cells, bool skip_multiline_non_zero_lines, ANSIBuf*);
 PyObject* line_as_unicode(Line *, bool, ANSIBuf*);